Liquid Electrodes Morph Into Flexible Wires for Neural Stimulation

Our nervous system is specialized to produce and conduct electrical currents, so it's no surprise that gentle electric stimulation has healing powers. Neural stimulation-also known as neuromodulation, bioelectronic medicine, or electroceuticals-is currently used to treat pain, epilepsy, and migraines, and is being explored as a way to combat paralysis, inflammation, and even hair loss. Muscle stimulation can also bestow superhuman reflexes and improve short-term memory.

But to reach critical areas of the body, such as the brain or the spine, many treatments require surgically implanted devices, such as a cuff that wraps around the spinal cord. Implanting such a device can involve cutting through muscle and nerves (and may require changing a battery every few years).

Now, a team of biomedical engineers has created a type of electrode that can be injected into the body as a liquid, then harden into a stretchy, taffy-like substance. In a paper in the journal Advanced Healthcare Materials, the multi-institutional team used their "injectrodes" to stimulate the nervous systems of rats and pigs, with comparable results to existing implant technologies.

"Instead of cutting down to a nerve, we can just visualize it under ultrasound, inject this around it, and then extrude back a wire to the surface," says study author Kip Ludwig, a professor of biomedical engineering and neurological surgery at the University of Wisconsin-Madison. That process creates a bypass between the surface of the skin and the deep nerve one wants to stimulate, without damaging tissue in between, he adds.

Researchers have created numerous flexible or stretchy electrodes to mold to the shape of, say, brain tissue, but this technology can be injected into the body and fill in cracks and crevices around nerves.

Working with Andrew Shoffstall at Case Western Reserve University and Manfred Franke of Neuronoff Inc., a California-based biotech company, Ludwig and colleagues developed an electrode consisting of bits of metal and a silicon base-similar to surgical glue-that combine to form a thick liquid. This liquid can be put into a syringe and injected into the space around a nerve, where it hardens into a solid form, with a consistency similar to taffy.

This taffy-like wire is conductive, can move and bend with the nerve or joint, and can be activated to stimulate the nerve with an inexpensive external unit-a transcutaneous electrical nerve stimulation, or TENS, unit-which anyone can buy at a pharmacy or online.

To test their new creation, the researchers injected the material into rats and pigs, and compared the performance to that of silver wires and a clinical electrode implant. The injectrodes worked just as well as both other tools, and even appeared to require a lower current for the same amount of neural activity. It is also possible to tailor the viscosity, or thickness, of the liquid electrodes for different applications, says Ludwig.

Illustration: Neuronoff This illustration shows how an "injectrode" consisting of a silicon base [1] and bits of metal [2] can be injected into the body as a liquid and then harden around a nerve to enable electrical stimulation.

Still, to remove the wire, one would have to "go in and get it," says Ludwig-meaning surgically remove it like any other electrical lead. Currently, his team is testing the safety and efficacy of the injectrodes over long periods of time and testing the possibility of having robots inject the material. Ludwig hopes to apply to the FDA and begin safety testing in humans in two years.

Ludwig and his collaborators co-founded Neuronoff to commercialize the technology. The team also recently received a US $2.1 million grant from the National Institutes of Health to test the injectrodes as an alternative to opioids for treating chronic back pain.